Where does shear collide?

The deformation or stress in a solid in which parallel layers slide past one another, so that a small square marked on the material is skewed into a parallelogram without changing its volume. Timoshenko and Goodier's 1970 Theory of Elasticity set out how shear stress, the force per unit area acting along a surface, relates to shear strain, the angle of that skewing, through the shear modulus of the material. The referent is an internal state of a loaded solid, measured in pascals, and it differs from a mapped body of sheared rock, a change of wind across the sky, or the sliding of layers inside a moving fluid.[1]

Unit

A shear stress in pascals, or a shear strain given as an angle of skew

Recovery means

In the elastic range the skew reverses when the load is removed; past the yield point some of it stays

Measured by

Found from applied load and geometry, or from strain gauges and torsion tests that give the shear modulus

A tabular region of rock that has taken up intense deformation while the blocks on either side moved past each other, whether by brittle slip along a fault or by ductile flow deep in the crust. Ramsay's 1980 review Shear zone geometry in the Journal of Structural Geology described how strain is distributed across such zones and how the deformed rock fabric records the direction and amount of movement. The referent is a mapped body of deformed rock in the Earth, spanning millimetres to kilometres, not a stress inside a small solid sample, a gradient of wind, or a flowing fluid.[2]

Unit

A band of deformed rock, described by its width and the amount of offset across it

Recovery means

Not applicable

Measured by

Mapped in the field and in thin section by the offset of markers and the orientation of the deformed rock fabric

A change in wind speed or direction over a short distance in the atmosphere, whether measured vertically between two heights or horizontally across a boundary. Weisman and Klemp's 1982 study in Monthly Weather Review showed how the vertical wind shear through a storm's depth controls whether convection stays as short-lived cells or organises into long-lived rotating storms, and the same gradient is a sharp hazard to aircraft near the ground. The referent is a gradient of wind through open air, measured as change in velocity per unit distance, unlike a stress in a solid, a body of sheared rock, or the internal sliding of a fluid along a surface.[3]

Unit

A difference in wind velocity per unit distance, vertical or horizontal

Recovery means

Not applicable

Measured by

Derived from wind profiles by radiosonde, wind profiler, Doppler radar, or aircraft as the change in wind between levels

The rate at which neighbouring layers of a flowing fluid slide past one another, called the shear rate, together with the drag this places on a bounding surface, called the wall shear stress. Malek, Alper, and Izumo's 1999 review in JAMA, Hemodynamic Shear Stress and Its Role in Atherosclerosis, described how blood flowing along a vessel exerts this wall shear stress and how the cells lining the artery detect and respond to it. The referent is a property of a moving fluid and the force it lays on a wall, measured as a shear rate per second or a stress in pascals, not a static stress in a solid, a deformed body of rock, or a wind gradient in the sky.[4]

Unit

A shear rate per second within the flow, and a wall shear stress in pascals

Recovery means

Not applicable

Measured by

Computed from the velocity profile of the flow, or estimated from flow rate and vessel geometry and by flow imaging